ERVW-1 and viral infectious disease: In particular, mutations in the MSD have been shown to have an effect on Env-mediated fusion such that mutations of conserved residues in the MSD, including the midspan arginine, led to decreased cell–cell fusion and decreased viral infection (Kondo et al., 2010; Shang & Hunter, 2010; Shang et al., 2008; Yue et al., 2009).